The use of coal for the production of energy is greatly impeded by the serious environmental factors associated with combustion of coal. The main problem is the discharge of acidifying substances such as sulphur and nitrogen oxides. Attempts have been made to a certain extent to solve this problem by various scrubbing steps but these entail a considerable increase in costs and it is extremely difficult or impossible with conventional technology to achieve the degree of purification which will be demanded if coal is to be accepted as a leading raw product for energy.
These problems can be solved by first gasifying the coal and then producing energy by combustion of the gas generated. It is relatively easy to achieve a high degree of purification, i.e. more than 95%, of sulphur in the reducing coal-gas and since the combustion is then of a gaseous fuel it can be arranged so that considerably less nitrogen oxide is formed than is possible with solid or liquid fuels. Gasification also offers better solutions to several other environmentally detrimental effects of coal combustion such as the discharge of mercury, polyaromatic hydrocarbons, heavy metals and flying ash.
Considerable effort has been made recently to develop the coal gasification method for producing energy, but in all cases costs have proved all too high. The main reason for this is the vast consumption of oxygen gas, in view of the high investment costs and relatively high consumption of electricity entailed in producing the oxygen gas. Furthermore in most coal-gasifiers 10-20% of the gas formed is burned in the gasification reactor in order to meet the heat requirement for gasification and to achieve a favourable reaction temperature.
Simple and inexpensive methods of manufacturing gas suitable for the production of energy are coal-gasification processes using air and consuming a minimum of coal. Coal substantially in lump form is gasified in counterflow with hot air-blast in the shaft furnace. The gas formed has a temperature of approximately 500.degree. C. and thanks to the low temperature, includes reasonable quantities of tar substances and small amounts of uncombusted coal in particle form.
As revealed in Swedish patent applications 85 04 439-4 and 85 04 440-2 same applicant, it has been proposed to thermically crack hydrocarbons occuring in a gas produced by gasification of coal, by supplying a gas heated by plasma-generator. After partial cracking, the gas is conducted through a dolomite filter of the type used in the Wiber-Soderfors process. Complete cracking of the remaining tar substances is obtained during transport through the filter, and the gas is purified from sulphur at the same time. One object of the invention is to further improve the technology proposed in the above-mentioned patent applications by further reducing the consumption of electricity.